I had the same problem when I tried to use morphix, lindows, mandrake and fedora core distros on my laptop. Only Knoppix boots properly on my laptop. I used to think that it could be due to a bad burn. Maybe I shouldn't have burned the ISO at 24x? But then I tried using those distro's to boot my desktop pc and they all seem to work just fine. My conclusion is, some distro's are just not compatible with certain systems such as the system your trying to boot it on. Try using the same CD to boot a different computer and see if it works. If it succeeds the problem could be your computer. If it fails , then its probably a bad burn so try burning the iso again at a lower speed and also check md5 to make sure you did not corrupt the file while downloading. Oh by the way, sometimes if you are able to get to the knoppix "boot:" screen, but have problems when the linux system is starting up (when hardware is being detected), it could be due to problems with scsi or usb. I had that problem with knoppix 3.4 and my laptop. I had to use the 'noscsi' cheat followed by the cheatcode to restart hotplug. Check the Docs section to get the cheatcodes and just try em really.

I hope this helps.